Thermoplastic polymeric materials are used extensively in automotive vehicles and for other purposes. The automotive industry uses tubes and hoses for transporting refrigerants and compressed air.
For automotive applications hoses must be resistant to chemical attack from road salts; exhibit high flexibility, and at same time, must have high burst pressure resistance. Polyamide 11 and 12 have good resistance to salt solutions such as zinc chloride solutions and flexibility; but are expensive materials. PA 6 and PA 66 are less expensive and have high strength but they are very sensitive to chemical attack by salt solutions, and lack adequate flexibility. As a result a multilayer construction of tubing typically is used to obtain the desired properties in automotive applications at an optimal cost.
Patent application EP 1378696 discloses a multilayer hose for compressed air comprising an inner layer made of PA 11 or PA 12, and outer layer made of a polyamide chosen from the same products as the inner layer and which can be the same or different, and at least one intermediate layer positioned between the inner layer and the outer layer chosen from blends of polyamide and polyolefin comprising a polyamide matrix, copolymers comprising polyamide blocks and polyether blocks, or blends of polyamides and of copolymers comprising polyamide blocks and polyether blocks.
Published U.S. Patent application 2009/0017247 discloses a multilayer hose for compressed air comprising: an outer layer made of polyamide (for instance PA12), optionally an intermediate layer of a polymer having a flexural modulus of less than 500 MPa, and inner layer of a polymer having a flexural modulus of greater than 1100 MPa (for instance PA6 or PA66). Intermediate layers are often used in the industry to obtain good bonding between two incompatible polyamide layers. However, the presence of intermediate layers result in an increase in the total number of layers in the multilayer hose or tubing and are economically less desirable than one requiring no bonding layers,
US Patent Publication 2010/0233402 entitled “Salt Resistant Polyamide Compositions” discloses certain semi-aromatic copolyamides that exhibit improved chemical resistance especially to metal halides and salts compared to corresponding aliphatic homopolyamides. In these copolyamides, at least 15 mole percent of the repeat units are derived from monomers that comprise an aromatic structure. Thus, semi-aromatic copolyamides 612/6T comprising 20 to 30 mole percent 6T units exhibit improved salt resistance than corresponding homopolyamide PA 612.
Needed are multilayer tubing or hoses that have a combination of good salt resistance, high burst resistance, high flexibility and excellent interlayer adhesion without the use of intermediate adhesion or tie layers.